The invention relates in general to direct tension indicating washers and in particular to direct tension indicating washers that limit creep.
Direct tension indicating (DTI) washers are used in the art to indicate when proper bolt tension has been reached. U.S. Pat. No. 5,931,618, the entire contents of which are incorporated herein by reference, discloses an exemplary DTI. FIG. 1 is a top view of the DTI from U.S. Pat. No. 5,931,618. A direct tension indicating washer 60 includes protuberances 12 formed on a first surface 14 and corresponding indentations 16 formed on a second surface 18 opposite the protuberances 12. The direct tension indicating washer 60 also includes channels 62 that lead from each indentation 16 to the outer diameter of the direct tension indicating washer 60. The indentation 16 is filled with an indicating material 64. The direct tension indicating washer 60 is manufactured using a tool and die to stamp the protuberances 12, indentations 16 and channels 62 into a blank washer. Other processes, such as metal machining, electronic printing, or metal casting may be used to form the direct tension indicating washer 60. Direct tension indicating washer 60 is made from carbon steel, but stainless steel, nonferrous metals, and other alloy products may also be used. The indicating material 64 is an extrudable, elastomeric solid material such as colored silicone. FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1.
While the direct tension indicating washer of FIGS. 1-2 is well suited for its intended purposes, a tension indicating device for compressive loads required by many non-structural industries (e.g., mining, gasketed pipe flange joints) is significantly lower than what the typical steel material for a structural DTI, (ASTM 959) is manufactured with. As such, the amount of plastic deformation required to provide the appropriate user feedback and other performance requirements of a tension indicating device are difficult to achieve with steel as a sole material of manufacture. Thermoplastics or other intrinsically softer materials are available to provide the required amount of deformation for the intended performance criteria, but such materials have long term compressive creep concerns that would exclude their suitability.